Sand and cement of Screed + Adhesive

I am drawing up proposal for 4000 sqm of polished sand stone for shopping pavilion 1st floor. As it stands there is reinforced concrete slab on steel work with a 50mm screed already in place. Could anyone advise on either adding another layer of screed plus adhesive or to go the way of bedding tiles on sand and cement with no need for and extra layer of screed. Any pros and cons would be gratefully appreciated